The immunoregulatory function of the B lymphocyte receptor for IgE (FcXIR) will be studied. Preliminary studies have indicated that the isolated B lymphocyte FcXIR will suppress the in vitro development of IgE producing plaque forming cells, indicating the potential immunoregulatory role of this molecule. In addition, physico-chemical studies have demonstrated that the B cell FcXIR can be, by limited proteolysis, be converted into carbohydrate containing fragments that are quite similar in size to T cell produced IgE-binding factors. Attempts will be made to produce an FcXIR+ cell line in the murine system via the fusion of B cells from Nippostrongylus brasiliensis (Nb) infected mice with enzyme deficient B cell lines. These FcXIR+ B cell lines and/or B cells isolated from Nb infected mice will serve as a source for the isolation of FcXIR preparations. These will then be studied for their immunoregulatory properties with regard to suppression or enhancement of the IgE and IgG responses in mice. In addition specific FcXIR fragments will be tested in the same manner and compared with the immunoregulatory IgE-binding components isolated from murine T hybridoma cells. The functional role of the FcXIR, while incorporated in the B cell membrane, will also be studied. Preliminary studies have suggested an interaction between FcXIR and B cell surface immunoglobulin (sIg). The modulatory effect that the interaction of FcXIR with sIg during B cell activation, as induced by sIg crosslinking, will be determined. In addition, membrane depolarization effects of B cell FcXIR crosslinking will be studied both in and of itself and in combination with sIg crosslinking.